Brasiliano (Pan-African) granitic magmatism in the Pajeú-Paraíba belt, Northeast Brazil: an isotopic and geochronological approach

Precambrian Research 135 (2004) 23–53

Brasiliano (Pan-African) granitic magmatism in the Paje´u-Para´ıba belt, Northeast Brazil: an isotopic and geochronological approach Ignez P. Guimar˜aesa,∗ , Adejardo F. Da Silva Filhoa , C´ıcera Neysi Almeidab , W.R. Van Schmusc , Jo˜ao M.M. Ara´ujoa , Silvana C. Meloa , Evenildo B. Melod a

c

Departament of Geology, Pernambuco Federal University, Recife, Brazil b CNPq DCR- Research, Brazil Department of Geology, 1475 Jayhawk Blvd.-Room 120, University Kansas, Lawrence, KS 66045, USA d Department of Mine Engineering, Pernambuco Federal University, Recife, Brazil Received 9 December 2003; accepted 7 July 2004

Abstract The Neoproterozoic Brasiliano/Pan-African Orogeny in the Borborema Province (BP) NE Brazil, is characterized by intense granitic magmatism spatially associated with continental scale shear zones and metamorphism under high temperature conditions. U/Pb zircon and Rb–Sr whole rock data for 14 granite intrusions from the Paje´u-Para´ıba Belt have ages that suggest more than 100 Ma of intrusive magmatism, which can be divided in four events: the oldest granites (between 620 and 600 Ma) are medium-to-slightly high-K calc-alkaline I-type granitoids, intruded into metagreywackes and gneiss-granites of 0.9–1.0 Ga; they are related to the peak of metamorphism and to the development of a flat lying foliation. The youngest intrusions (540–520 Ma) have geochemical signature of A-type, post-orogenic, extension-related granites, and are associated with sub-volcanic bimodal magmatism, probably contemporaneous with deposition of small sedimentary basins in the North (Iara, Jaibaras graben and Sa´ıri) and Central (Fatima, Betˆania and Carna´ubeira) Tectonic Domains of the Borborema Province; they reflect post-tectonic relaxation of the Brasiliano Orogeny. Between these episodes, two other intrusive events were identified: (1) high-K calc-alkaline granitoids and shoshonitic granitoids associated with mafic syenites, meladiorites and hornblende – biotite diorites, intruded between 590 and 581 Ma, associated with a transcurrent deformation event, and (2) alkaline post-collisional granitoids having U/Pb zircon ages of ca. 570 Ma, marking the final stage of the Brasiliano Orogeny. In the South Domain of the Borborema Province, migmatization took place between 610 and 600 Ma. Although similar ages were not found in the granitoids of the Central Tectonic Domain, field evidence suggest that migmatization followed the intrusions of the oldest (620–600 Ma) granitoids. © 2004 Elsevier B.V. All rights reserved. Keywords: Neoproterozoic; Granitoids; Crustal evolution; Borborema Province

∗

Corresponding author. E-mail address: [email protected] (I.P. Guimar˜aes).

0301-9268/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.precamres.2004.07.004

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1. Introduction The Borborema Province (BP) comprises a large region in northeastern Brazil, north of the S˜ao Francisco Craton (Fig. 1A). In pre-drift reconstructions, this province is adjacent to similar Pan-African belts and cratonic terranes in western Africa (Caby et al., 1981; Caby, 1989; Jardim de S´a, 1984; Toteu et al., 1990, 1994, 2001; Brito Neves and Cordani, 1991; Castaing et al., 1993; Trompette, 1997; Brito Neves et al., 2002; Neves, 2003). The BP represents the western part of a belt that occupies northern Gondwana (Van Schmus et al., 1995). The late Neoproterozoic (Brasiliano = Pan-African) evolution of the Borborema Province, northeast Brazil, was marked by a great abundance of granitic intrusions, the majority of them, associated with NE–SW shear zones (Vauchez et al., 1995; Neves and Vauchez, 1995; Archanjo, 1993; Jardim de S´a, 1994). Almeida et al. (1967), based on petrography, recognized four granite types within the Borborema Province: (1) Conceic¸a˜ o type—medium to fine grained granodiorites and tonalites; (2) Itaporanga type—granodiorites with large K-feldspar phenocrysts; (3) Itapetim type—fine grained biotite granites, associated with the Itaporanga type, and (4) Catingueira type—peralkaline granites, syenites and quartz syenites. Sial (1986) characterized geochemically the granitoids of the Pianc´o Alto Br´ıgida Fold Belt in the Central Tectonic Domain of the Borborema Province (=Cachoeirinha–Salgueiro Belt) and correlated them with the granitoids described by Almeida et al. (1967), i.e.: (1) Calc-alkalic (Conceic¸a˜ o type); (2) Potassic–calc-alkalic (Itaporanga type); (3) Peralkalic (Catingueira type) and (4) Trondhjemitic (Serrita type). Brito Neves et al. (2000), based on previous studies, divided the Brasiliano granitoids into three super suites:

(I) Composed of hybrid and crustal granitoids (calcalkaline, high-K calc-alkaline, trondhjemitic and peraluminous suites) intruded since the contractional tectonics, up to the late phase of strike-slip displacements. (II) Enriched-mantle derived suite. This includes high-K calc-alkaline, shoshonitic and ultrapotassic, and alkaline suites, syn- and late kinematic intrusives of the major stike-slip events. (III) Within-Plate hybrid suites, related to post-closure uplift and the collapse phase of the orogenic structures. We report here the results of our petrologic, geochemical and isotopic study of 14 Neoproterozoic granitic intrusions from the central part of the BP, and discuss their origin in terms of magmatic and tectonic processes. 2. Geological setting The BP consists of gneissic and migmatitic basement complexes, mostly formed during the Paleoproterozoic (Transamazonian tectonic cycle ∼ 2.0–2.2 Ga), and are partially covered by Mesoproterozoic to Neoproterozoic metasedimentary and metavolcanic rocks (Van Schmus et al., 1995; Dantas et al., 1998; Fetter, 1999; Brito Neves et al., 2001; Kozuch, 2003). In addition to the Transamazonian orogenic cycle, the BP was affected by the Cariris Velhos (∼1.0 Ga) and Brasiliano (0.6 Ga) events. The Cariris Velhos event is represented by muscovite–biotite gneisses, garnet-biotite schists, and metavolcanic rocks intruded by granitic plutons (now augen-gneisses) of early Neoproterozoic age (Santos, 1995; Brito Neves et al., 2001; Kozuch, 2003); it is mainly distributed in the Central Tectonic Domain of the BP (Fig. 1B). The Brasiliano event affected the entire province and was responsible

Fig. 1. (A) The mainly shear zones and the tectonostratigraphic terranes proposed by Santos et al. (1999): AMT, Alto Moxot´o; APT, Alto Paje´u; RCT, Rio Capibaribe; PABB, Pianc´o Alto Br´ıgida belt; SB, Sergipano belt; PAT, Pernambuco–Alagoas; GJT, Grajeiro; SED, Serid´o belt; JC, S˜ao Jos´e do Campestre; RP, Rio Piranhas; JG, Jaguaribeano; CE, Ceara; MC, Medio Corea´u; RP, Riacho do Pontal. WPSZ and EPSZ are the two branches (west and east) of the Pernambuco lineament as proposed by Neves and Mariano (1997). PSZ, Patos shear zone. The occurrences of eclogite core from Beurlen et al. (1992). (B) Sketch geological map of the Central Tectonic Domain of the Borborema Province emphasizing the studied granitoids (1 = Timba´uba; 2 = Bom Jardim; 3 = Toritama; 4 = Fazenda Nova; 5 = Itapetim; 6 = Tabira; 7 = Paje´u; 8 = Solid˜ao; 9 = Campina Grande; 10 = Queimadas; 11 = Serra Branca; 12 = Prata; 13 = Plutons along the Afogados da Ingazeira Shear Zone (AISZ) – Pereiro, Serra do Velho Zuza). CCSZ = Campina Grande shear zone; SSZ = Solid˜ao shear zone; JBSZ = Juru – Bel´em shear zone.

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for low-to-high grade metamorphism, abundant magmatism, and development of continental-scale transcurrent shear zones. The most important shear zones are represented by the Patos and Pernambuco systems (Vauchez et al., 1995) and the Afogados da IngazeiraGalante-Mari shear zone (Brito Neves et al., 2001) (Fig. 1B). The E-W shear zones divide the BP into three segments, referred to as North, Central and South Tectonic Domain by Van Schmus et al. (1995). The studied area is located in the Central Tectonic Domain, which was previously named the Transversal Zone by Ebert (1970). It is limited by the Patos shear zone to the north, Pernambuco shear zone to the south, Afogados da Ingazeira shear zone to the west, and the coastal area to the east (Fig. 1B). Trompette (1994) suggests continuation of the Central Tectonic Domain to the African side of West Gondwana, between the Adamaoua and Garoua shear zones in Cameroun. However, structural data of Neves and Mariano (1999) indicate that the Pernambuco lineament is not a transcontinental structure. It is segmented into two branches and its tectonic role during the Brasiliano was secondary. In the past few years, many authors (Santos, 1995; Santos et al., 1997; Ferreira et al., 1998; Santos and Medeiros, 1999) suggested that the Brasiliano evolution of the BP was controlled by accretion of exotic terranes (Fig. 1A). Each domain was segmented into many terranes. Brito Neves et al. (2000, 2001) kept the terrane model only for the Central Tectonic Domain. The South Tectonic Domain was divided into Southern Domain and Pernambuco Alagoas Massif. The later has been called the Pernambuco-Alagoas complex by Da Silva Filho et al. (2002). In the Southern Domain, Brito Neves et al. (2000) kept the tectonic model of Brito Neves (1983), dividing it into: Sergipano fold belt, Rio Preto fold belt, and Riacho do Pontal fold belt. The North Tectonic Domain was subdivided into: Rio Grande do Norte domain, encompassing two-fold belts (Jaguaribeano-Encanto and Serid´o), two massifs (S˜ao Jos´e do Campestre and Rio Piranhas), Central Cear´a domain, and M´edio Coreau domain. The Central Tectonic Domain encompasses: Pianc´o-Alto Brig´ıda (Pianc´o Alto Br´ıgida Belt), the Alto Moxot´o (AMT), Alto Paje´u (APT), and Rio Capibaribe (RCT) terranes. According to this terrane accretion model, the studied granitoids are located in the APT, AMT and RCT

(Fig. 1A and B), which are terranes in the former Paje´uPara´ıba fold belt (Brito Neves, 1983). The APT comprises muscovite–biotite gneisses, garnet-biotite schists, and metavolcanic rocks intruded by early Neoproterozoic granitic plutons (now augengneisses). These rocks were deformed during the Brasiliano cycle, initially by a transcurrent episode, and later by extension (Santos et al., 1997; Brito Neves et al., 2001; Neves, 2003). The AMT is composed of metavolcanometasedimentary sequences, including a calc-alkaline volcanic sequence of arc affinity and Paleoproterozoic blocks (2.1–2.4 Ga) of tonalitic to granodioritic composition (Santos, 1995). The RCT is constituted by early Neoproterozoic sequences of schist and gneiss with intercalations of marble and calc-silicate rocks plus Mesoproterozoic orthogneiss of granitic composition as well as anorthositic intrusions. Some authors (Neves and Mariano, 1997; Mariano et al., 2001), based on geochemical and structural data, concluded that the Pernambuco shear zone can not be considered as the limit between distinct terranes as proposed by Santos (1995) and Brito Neves et al. (2001); in addition, the distinct terranes forming the BP are underlain by lithospheric mantle blocks, with similar geochemical and isotopic signature. According to these authors, these are strong arguments against the evolution of the BP based on the terrane accretion model. On the other hand, structural, geophysical and isotopic data suggest that the Patos shear zone is the limit between the APT and the Rio Grande do Norte Domain (Brito Neves et al., 2001). Beurlen et al. (1992) described two occurrences of ophiolites (Fig. 1B) within Paleoproterozoic gneisses, close to their tectonic contact with early Neoproterozoic metasedimentary rocks of the Pianc´o Alto Br´ıgida belt. These occurrences have been cited as evidence of oceanic closure during either the Brasiliano (Bittar and Campos Neto, 2000; Beurlen et al., 1992) or the Cariris Velhos (Santos et al., 1997) orogeny. In this work, we will follow the tectonic model of Brito Neves (1983), in which the studied area is part of the Paje´u-Para´ıba belt, without further discussion on this always controversial subject of terrane nomenclature. Bittar (1999) estimated Brasiliano metamorphic peak conditions for the west part of the APT (S˜ao Cae-

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tano Complex) at P = (4.4 ± 1.0) kbar (geobarometer of Hodges and Spear, 1982) and T = 700 ◦ C (geothermometer of Spear, 1981). These data are similar to those obtained by Coutinho (1994) for the garnetbiotite gneisses from the S˜ao Caetano Complex, based on the equilibrium of the metamorphic assemblage, and using the Thermocalc program (P = (4.7 ± 1.6) kbar, T = (703 ± 101) ◦ C) and those obtained by Leite et al. (2000) for the APT in the Monteiro-Sum´e area. It is important to establish the timing of the peak of the metamorphism in order to assess its possible relationship with the plutonism. However, this has not been done systematically in the Central Tectonic Domain of the Borborema Province. Leite et al. (2000) reported an upper intercept U–Pb zircon age of 972 ± 4 Ma for orthogneisses intruded by the Brasiliano Tabira pluton and a concordant sphene fraction from the same sample giving the age of metamorphism at 612 ± 9 Ma. In north-central Cameroon the peak of granulite facies metamorphism, associated with flat-lying foliation, has ages in the range of 620–630 Ma (Toteu et al., 1987, 1994). The ages can also represent the age of the metamorphic peak in the studied area, if a preGondwana reconstruction is considered. Because the major Brasiliano granitic intrusions are associated with shear zones, their study is important to understand the evolution of the BP during the Brasiliano. In the western part of the Paje´u-Para´ıba Belt, Sales (1997) identified two distinct levels of crustal blocks separated by the Afogados da Ingazeira shear zone (AISZ—Fig. 1). In the south, the present level of exposure reflects metamorphism under lower pressure and temperature conditions (4 kbar and 600 ◦ C) when compared to the north (6.4 kbar and 670 ◦ C). Vertical movement of blocks along the AISZ is also supported by structural data (Sales, 1997; Ara´ujo, 1997). Distinct crustal blocks are also recorded in the northeastern part of the Paje´u-Para´ıba belt: the block located south of the Campina Grande shear zone (which is part of the Patos Lineament), shows intense migmatization and metamorphic conditions under pressure higher than 6 kbar and temperature of ca. 700 ◦ C (Almeida et al., 1997). Sm–Nd TDM model ages in rocks of this block are ca. 2.0 Ga. The block to the north of the Campina Grande shear zone is composed of well foliated biotite ± muscovite orthogneisses, metamorphosed under amphibolite facies conditions, with Sm–Nd model ages in the 1.5–1.4 Ga range (Brito Neves et al., 2001).

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Narrow and elongated deposits of detrital sediments, sandstones, arkoses and conglomerates occur to the west of the AISZ. These sediments have been interpreted as part of the Tacaratu Formation of the Jatob´a Basin, of Upper Silurian age (Veiga and Ferreira, 1990). However, elongated and narrow basins are rift-related, and the Jatob´a Basin is interpreted as a syncline from the Paleozoic. Other small Paleozoic basins occur in the Central Tectonic Domain (Betˆania, F´atima, Carna´ubeira, Mirandiba, S˜ao Jos´e do Belmonte—Veiga and Ferreira, 1990). The sediments cropping out in all of these small basins have been interpreted as chrono-correlated to the Tacaratu Formation. The absence of fossils in these sediments makes it difficult to date them. The Upper Silurian age was estimated from lithologic correlation. Several Cambrian pullapart basins (Iara, Jaibaras graben and Sa´ıri), consisting of extension-related molassic deposits, have been described further northwest in Cear´a State (Fetter, 1999). The Mocambo granite that flanks the Jaibaras Graben, the largest of these extensional basins, yields a U/Pb zircon crystallization age of 532 ± 6 Ma (Fetter, 1999). Bimodal sub-volcanic rocks occur as dyke swarms, in the Monteiro–Sum´e area, oriented in a general N-S direction. They are dacite and rhyolite with subordinate alkaline diabase and are co-magmatic with the Cambrian Prata Granitic Complex (Guimar˜aes et al., 2000).

3. The granite intrusions Twelve large granitoid intrusions (Queimadas pluton, Bom Jardim complex, Toritama complex, Timba´uba pluton, Tabira pluton, Itapetim complex, Fazenda Nova complex, Campina Grande Pluton; Serra Branca pluton, Solid˜ao pluton, Paje´u pluton, Prata complex) and two small plutons intruded along the Afogados da Ingazeira shear zone (Pereiro and Serra do Velho Zuza plutons) were investigated for their main field, petrographic geochemical and isotopic characteristics. The geochemical data from Fazenda Nova are from Neves and Vauchez (1995). Except for the Bom Jardim and Toritama complexes, which are syenites, all the other intrusions are granite or granodiorite to tonalite in composition. Most have K-rich dioritic enclaves. The Tabira and Timba´uba plutons comprise EW elongated intrusions of porphyritic to equigran-

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ular epidote-bearing biotite-hornblende granodiorites to monzogranites, deformed under high-T conditions. Rounded to elliptical microgranular enclaves are common. Amphibole-rich clots, surrounded by coarsegrained amphibole and biotite are hosted by the granodiorite-monzogranite. “chunks of quartz”, up to 30 cm long, were also observed enclosed by the granitoids of the Timba´uba Pluton. Amphibole-rich clots and “chunks of quartz” are features shared by many granitoid intrusions of similar ages and compositions in other belts of the Borborema Province (Guimar˜aes and Da Silva Filho, 2003; McReath et al., 1993; Sial et al., 1998). The Timba´uba pluton is intruded into the contact between Cariris Velhos (1.0–0.95 Ga) orthogneissmetagreywacke and Neoproterozoic metasedimentary sequences (Gomes, 2001); the latter include garnetbearing biotite gneiss and limestone with Nd model ages (TDM) between 1.5 and 1.4 Ga range. The Tabira pluton intrudes ca. 0.95 Ga orthogneiss (Leite et al., 2000) with Nd model ages in the 1.5–1.4 Ga range (Kozuch et al., 1997). Flat-lying foliation cut by late, high-angle foliation is recorded in the plutons and in their country rocks, suggesting that the emplacement of these granitoids is related to the peak of regional metamorphism and associated with the flat-lying event. Evidence of deformation under high-T conditions recorded within the Timba´uba pluton, dikes of dioritc composition associated with the Timba´uba pluton showing migmatization, and the intrusion of the Timba´uba Pluton during a plastic stage of the country rocks (Fig. 2) suggest that the Timba´uba intrusion was pre- to syn-migmatization. The Itapetim complex cuts Cariris Velhos orthogneisses (∼950 Ma; Kozuch et al., 1997) and metassediments with TDM ∼ 1.4 Ga. The ca. 1.0 Ga orthogneisses have flat-lying foliation and are locally migmatized. The Itapetim complex is constituted by epidote-bearing porphyritic monzogranites, with large perthite and plagioclase phenocrysts (up to 7 cm long) in a matrix of biotite, hornblende, microcline plagioclase, and quartz. Swarms of diorite enclaves are quite common. Late dikes of granodioritic composition, showing magmatic foliation and layering, cut the complex.

The Bom Jardim complex intrudes migmatite with Sm–Nd model ages (TDM) of ca. 2.2 Ga, close to the contact with Mesoproterozoic supracrustal rocks. The Toritama complex intrudes 1.5 Ga orthogneissic migmatites (S´a et al., 2002) and Neoproterozoic paragneisses. Plutons of the complexes range in composition from hornblende-biotite ± clinopyroxene monzonite to syenite. They are very coarse to medium grained, and range in texture from porphyritic to equigranular. Enclaves of mafic syenite and hornblende-biotite diorite are common. The dominant facies is a mafic porphyritic monzonite to syenite, with phenocrysts of perthitic microcline in a medium-grained matrix composed of hornblende and locally contain relicts of clinopyroxene, biotite, microcline, plagioclase, small amounts of quartz, sphene, zircon, apatite, allanite, and monazite. Both intrusions show low-to-moderate inward dipping magmatic foliations. The Fazenda Nova complex, which is part of the Fazenda Nova – Serra da Japecanga Batholith (Neves et al., 2000), and the Campina Grande complexes are composed of coarse-grained porphyritic granites intimately associated with hornblende-biotite diorites. The porphyritic granite consists of megacrysts of perthitic microcline up to 10 cm long in a medium-grained matrix of plagioclase, quartz, microcline, hornblende, biotite; the accessory phases are sphene, allanite, zircon, apatite, and magnetite. Primary epidote was recorded in the coarse grained granites from the Campina Grande complex. The mafic facies occur as enclaves with a wide range of sizes. They are dioritic to granodioritic, varying in texture from fine-grained equigranular to porphyritic. The Fazenda Nova complex is mainly intruded into the contact zone between a 2.0 Ga sequence of orthogneisses and migmatized paragneisses and 1.5 Ga orthogneisses (S´a et al., 2002). The Campina Grande complex is intruded into the contact zone between migmatites with TDM ∼ 2.0 Ga in the south and early Neoproterozoic (0.95 Ga) orthogneisses in the north. The Paje´u complex intrudes early Neoproterozoic supracrustal rocks (Santos, 1995) and it is comprised of biotite-hornblende bearing, medium-grained

Fig. 2. (A) Deformation under high temperature and localized migmatization within the Timba´uba granites; (B) Diorite dyke associated to the Timba´uba intrusion, partially migmatized intruded during a plastic stage of the country rock; (C) Migmatized and folded diorite within the northern border of the Timba´uba intrusion; (D) Apophysis of the Timba´uba granites intruded during a plastic stage of the country rocks.

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syenogranite and coarse-grained porphyritic monzogranites. Diorite occurs as enclaves within the coarsegrained monzogranite. The Queimadas and Solid˜ao intrusions are comprised of, respectively, biotite-hornblende monzogranites and clinopyroxene-hornblende monzogranites to granodiorites; they also contain rare mafic enclaves. Both plutons had their emplacement controlled by shear zones and show magmatic fabrics overprinted by solid-state deformational fabrics. The Queimadas Pluton intrudes Paleoproterozoic migmatized sillimanite gneisses, with TDM ∼ 2.1 Ga (Van Schmus et al., 1995). It is a very large E-W trending dyke, cut by a later NEtrending ductile–brittle fault system (Almeida, 1999), that gave the body a mega-boudin shape. Its emplacement is considered to be syn-tectonic to the dextral Campina Grande shear zone. The Solid˜ao pluton intrudes early Neoproterozoic orthogneisses and supracrustal rocks. Its emplacement was initially connected to the kinematics and geometry of the dextral EW-trending Solid˜ao shear zone, which forms a conjugate pair with the sinistral NE-trending Afogados da Ingazeira shear zone. The conjugate shear zones created an extensional zone that allowed the emplacement of the Solid˜ao Pluton (Ara´ujo, 1997). The Serra Branca Pluton is composed of biotite granodiorites to monzogranites. Magmatic foliation is common as well as cross-bedding. The pluton appears to have evolved through distinct magma pulses, associated with the movement of the E-trending, dextral Coxixola shear zone. It is enclosed by Paleoproterozoic gneiss-migmatites with a Sm–Nd TDM model age of ca. 2.7 Ga (Guimar˜aes and Da Silva Filho, 1997). The Prata complex comprises two segments separated by a body of norite, which occurs as enclaves in the complex. The southern segment is composed of locally garnet-bearing, undeformed, equigranular to porphyritic biotite syenogranites, showing mixing and comingling with diabase and dacite (Melo et al., 1995). Allanite is the most abundant accessory phase; the crystals can reach up to 2 mm in length. Granitoids in the northern segment have a ubiquitous magmatic foliation. They are medium-to-coarse-grained hornblendebiotite monzogranites. Mafic enclave swarms are common. Rapakivi texture occurs locally. The whole complex cuts Paleoproterozoic migmatites. The Pereiro and Serra do Velho Zuza plutons show roughly rounded shapes and intrude migmatites with the age of 2.0 Ga.

The plutons consist of biotite syenogranites which contain some mafic enclaves. Biotites from Pereiro and Serra do Velho Zuza Plutons as well as those from the granites of the Prata Complex are rich in annite, suggesting crystallization under low fO2 conditions.

4. Geochronological data U/Pb zircon analyses were performed at the Isotopic Geochemistry Laboratories, Geology Department, Kansas University, USA. U–Pb zircon data were obtained from three or four multicrystal (at least four crystals) magnetic fractions from each intrusion. The procedures are described in Appendix A. The zircon grains were abraded for 2 h and then washed with HNO3 . Those grains clear of inclusions were picked to be analyzed. At least two distinct zircon populations were recorded in the Itapetim complex, Timba´uba and Tabira plutons. The analyzed zircon population consists of euhedral, internally clear zircon grains free of inclusions. The results are shown in Table 1 and Fig. 3A–C. The discordia yield upper intercept ages ranging from 645 ± 4.8 Ma (Timba´uba), 638 ± 4.9 Ma (Itapetim), to 624 ± 2.1 Ma (Tabira), when forced to zero. Kozuch (2003) obtained concordant zircon fraction from one sample of the Tabira granodiorite, with the age of 611 ± 9 Ma, and an average of three age determinations of 604 ± 15 Ma, which could be considered identical within error to the regressed age presented here. Ages in the same time span, have been obtained by Kozuch (2003) in other granitoids with similar petrographic and geochemical features (Olho D’agua granodiorite with an age of 607 ± 7.7 Ma) and gabbros (Alto Vermelho, age of 619 ± 9 Ma) in the Alto Br´ıgida fold belt and gabbros from the Paje´u-Para´ıba belt (Jabitac´a, age of 626 ± 3 Ma). The age of 612 ± 9 Ma obtained by Leite et al. (2000) for the metamorphism of the Tabira pluton country rocks is similar to the crystallization age of the Tabira granodiorites, suggesting a chronologic correlation between the intrusion of the Tabira granodiorites and the metamorphism. Only one zircon population was recorded so far within the Bom Jardim, Paje´u and Campina Grande complexes. The zircons are euhedral, pink, clear grains and gave upper intercept ages of 592 ± 7.4 Ma (Bom Jardim pluton), 586 ± 21 Ma (Paje´u Pluton) and 581

Table 1 U–Pb zircon data from the studied granitoids Sample fraction

Weight U (ppm) Pb (ppm) 206 Pb/204 Pb (mg) (observed)

206 Pb/238 U

±2σ (%)

207 Pb/235 U

±2σ (%)

207 Pb/206 Pb

±2σ (%)

206 Pb/238 U

±2σ (Ma)

207 Pb/235 U

±2σ (Ma)

207 Pb/206 Pb

±2σ (Ma)

55.81 360.05 46.06

5.8 39.4 5.1

1734 1317 1464

0.09580 0.10169 0.10054

0.57 0.56 0.64

0.81336 0.85960 0.85009

0.62 0.59 0.67

0.06157 0.06131 0.06135

0.22 0.18 0.20

589.8 624.3 617.4

3 3 4

604.3 629.9 624.7

4 4 4

659.4 650.1 651.4

1 1 1

Tabira M(−1) 0.14 M(0) 0.08 NM(−1) 0.05

47.83 111.29 734.54

5.1 12.1 75.2

2254 3684 1834

0.09904 0.09184 0.09876

0.49 0.54 0.51

0.82660 0.76725 0.82546

0.60 0.58 0.53

0.06053 0.06059 0.06062

0.22 0.21 0.13

608.8 566.4 607.2

3 3 3

611.7 578.2 611.1

4 3 3

622.7 624.7 625.7

1 1 1

Itapetim complex (see Guimar˜aes and Da Silva Filho, 2000 for data) Bom Jardim complex (sample BJ-275) 0.17 24.3 2.5 M(6)a 0.01 64.0 10.8 M(1)a 0.04 83.0 14.1 M(0)a 0.29 17.9 1.9 M(0)b M(−1)a 0.034 31.8 3.4

2598 2041 1945 4215 2114

0.09284 0.09432 0.09430 0.09437 0.09435

0.57 1.1 0.49 0.87 0.58

0.76190 0.77652 0.77462 0.77400 0.77742

0.59 1.1 0.50 0.88 0.60

0.05952 0.05971 0.05957 0.05968 0.05976

0.15 0.16 0.11 0.17 0.12

572.3 581.0 580.9 579.5 581.1

3 6 3 5 3

575.1 583.5 582.4 582.0 584.0

3 6 3 5 3

586.2 593.1 594.9 588.2 591.9

1 1 1 1 1

Paje´u complex (PJ-09) M(2) 0.023 299.6 M(0) 0.020 42.0 M(1) 0.021 437.0

28.7 4.1 41.9

4590 1584 3519

0.09116 0.09178 0.09005

0.52 1.03 0.50

0.74689 0.75327 0.73838

0.53 1.05 0.52

0.05942 0.05952 0.05947

0.11 0.18 0.13

572.3 581.0 555.6

3 6 3

566.4 570.1 561.5

3 6 3

582.7 586.2 584.2

1 1 1

Fazenda Nova complex (FN-01) M(1) 0.009 28.5 3.1 M(0) 0.017 1046.3 110.4 M(−1) 0.041 1004.1 108.9

816 3494 4320

0.09361 0.09283 0.09339

0.70 0.47 0.51

0.76731 0.76135 0.76700

0.80 0.49 0.52

0.05945 0.05948 0.05957

0.34 0.13 0.10

576.8 572.2 575.5

4 3 3

578.2 574.8 578.0

5 3 3

583.6 584.9 587.8

2 1 1

I.P. Guimar˜aes et al. / Precambrian Research 135 (2004) 23–53

Timba´uba M(−1) 0.11 M(0) 0.10 NM(−1) 0.16

31

32

Table 1 (Continued ) Sample fraction

Weight U (ppm) Pb (ppm) 206 Pb/204 Pb (mg) (observed) 6993 5932 8018 7651

±2σ (%)

207 Pb/235 U

±2σ (%)

207 Pb/206 Pb

±2σ (%)

206 Pb/238 U

±2σ (Ma)

0.09195 0.09122 0.09275 0.09243

0.53 1.07 0.49 0.80

0.75249 0.74971 0.75897 0.75731

0.55 1.34 0.51 0.81

0.05993 0.05960 0.05935 0.05943

0.16 0.80 0.13 0.10

567.0 562.8 571.8 569.9

3 6 3 5

570.0 568.0 573.4 572.5

3 8 3 5

580.2 589.3 579.9 582.7

1 5 1 1

207 Pb/235 U

±2σ (Ma)

207 Pb/206 Pb

±2σ (Ma)

Queimadas Pluton (see Almeida et al., 2002 for data) Serra Branca (SB-03) M(2) 0.05 M(1) 0.06 M(0) 0.04 M(−1) 0.07

252.5 622.5 126.6 291.7

22.8 51.1 11.2 23.4

1485 1547 924 1547

0.08527 0.07294 0.08106 0.07763

0.52 0.55 0.58 0.82

0.69720 0.59265 0.66348 0.63352

0.53 0.57 0.61 0.83

0.05930 0.05893 0.05936 0.05918

0.11 0.15 0.16 0.14

527.5 453.8 502.5 481.9

3 2 3 4

537.1 472.6 516.7 498.3

3 3 3 4

578.1 564.5 580.3 573.8

1 1 1 1

Solid˜ao Pluton (CS-07) M(1) 0.14 137.6 M(0) 0.01 1426.3 M(−1) 0.09 130.3 NM(−1) 0.09 138.1

23.9 196.6 14.9 19.3

2779 1450 1118 2103

0.16875 0.13430 0.11114 0.13844

0.51 0.48 0.49 0.51

2.32255 1.58349 1.11497 1.68402

0.52 0.50 0.51 0.89

0.09982 0.08552 0.07341 0.08822

0.09 0.10 0.13 0.72

1005.2 812.3 679.3 835.8

5 4 2 4

1219.2 963.7 883.0 1002.5

6 5 4 9

1620.8 1327.3 1199.9 1387.3

1 1 2 10

Serra do Velho Zuza Pluton (SVZ-250) M(2) 0.028 4230.2 328.5 M(1) 0.025 2857.8 213.9 M(0) 0.023 3280.6 281.1 M(−1) 0.025 2251.7 194.2

2256 2685 4040 4161

0.07379 0.07212 0.08250 0.08145

0.72 0.48 0.76 0.55

0.59346 0.57612 0.66386 0.65453

0.73 0.49 0.77 0.69

0.05833 0.05793 0.05836 0.05828

0.13 0.11 0.08 0.41

459.0 448.9 511.0 504.8

3 2 4 3

473.1 462.0 517.0 511.3

3 2 4 3

542.0 527.2 543.3 540.4

1 1 0.4 2

Pereiro Pluton (PE-01) M(3) 0.024 1659.4 M(0) 0.050 52.8 M(−1) 0.010 163.7

1314 4646 1024

0.06853 0.08162 0.08364

0.71 1.02 1.04

0.55307 0.65671 0.67427

0.82 1.07 1.10

0.05854 0.05835 0.05856

0.42 0.30 0.33

427.3 505.8 517.8

3 5 5

447.0 512.6 523.3

4 5 6

550.0 543.0 547.3

2 2 2

a b c

Abraded for 2 h. Abraded for 10 h. Abraded for 15 h.

121.0 4.5 15.0

I.P. Guimar˜aes et al. / Precambrian Research 135 (2004) 23–53

Campina Grande complex (CG-26A) 0.049 919.1 90.0 M(3)a 0.042 983.7 96.0 M(1)a 0.057 90.5 8.9 M(0)a M(−1)c 0.026 884.3 87.8

206 Pb/238 U

I.P. Guimar˜aes et al. / Precambrian Research 135 (2004) 23–53

33

Fig. 3. U–Pb Concordia diagram for the Timba´uba, Itapetim and Tabira intrusions.

± 2 Ma (Campina Grande Pluton) when forced to zero (Table 1; Fig. 4A and B). These data agree with Rb–Sr isochron ages of 592 ± 49 Ma for the Paje´u pluton (Guimar˜aes et al., 1999) and of 585 ± 38 Ma for the Bom Jardim and Toritama complexes (Guimar˜aes and Da Silva Filho, 1998). The Fazenda Nova complex, which constitutes the eastern part of the Caruaru–Arcoverde batholith (Melo et al., 2000), shows at least two distinct zircon populations. The euhedral, pink, clear zircon population defines an upper intercept age of 588 ± 12 Ma (Table 1,

Fig. 4C). This age is similar, within error, to those obtained by Pb–Pb zircon evaporation (Melo et al., 2000) in the central part of the Caruaru–Arcoverde batholith (591 ± 5 Ma). The Queimadas, Serra Branca, and Solid˜ao Plutons have similar ages, at ca. 570 Ma (Table 1 and Fig. 5A–C). The Queimadas and Serra Branca granitoids show at least two zircon populations. The younger populations have ages of 570 ± 24 Ma and 575 ± 16 Ma, respectively, when forced to zero. The Solid˜ao granitoid has a minimum age of 570 ± 21 Ma. This age

34

I.P. Guimar˜aes et al. / Precambrian Research 135 (2004) 23–53

Fig. 4. U–Pb Concordia diagram for the Bom Jardim, Campina Grande, Fazenda Nova, Paje´u plutons.

is similar to that (1991 ± 61 Ma, U.I.; 574 ± 74 Ma, L.I.) obtained by Kozuch (2003). There is a large inherited component in the rock that has an upper intercept of 2050 ± 62 Ma (Fig. 5C), which is similar to the Sm–Nd TDM model ages (2.14–2.07 Ga, Guimar˜aes et al., 1998). This suggests that the zircon grains in these granitoids best represent xenocrystic components inherited from the source rocks of the Solid˜ao granitoids. The Serra do Velho Zuza and Pereiro plutons show at least two zircon populations. The light pink, euhedral one gave ages of 538 ± 23 Ma (four fractions) and 544

± 6.7 Ma (three fractions), respectively (Fig. 6A and B; Table 1). The southern segment of the Prata Complex shows a Rb–Sr isochron age of 512 ± 30 Ma (Melo et al., 1995). These crystallization ages allow the studied granitoids to be divided into four groups. Group 1 has ages in the 640–610 Ma range (Tabira, Timba´uba, Itapetim). Group 2 has ages of 590–580 Ma (Bom Jardim, Toritama, Paje´u, Fazenda Nova, Campina Grande). Group 3 has ages of ca. 570 Ma (Queimadas, Solid˜ao, Serra Branca). Group 4 has ages in the 512–545 Ma range (Prata, Pereiro, Serra do Velho Zuza).

I.P. Guimar˜aes et al. / Precambrian Research 135 (2004) 23–53

35

Fig. 5. U–Pb Concordia diagram for the Queimadas, Serra Branca, Solid˜ao plutons.

5. Geochemistry Major and trace element data of representative samples are given in Table 2. The granitoids of Groups 2–4 are K-rich rocks (Fig. 7) with K2 O/Na2 O ratios > 1, while granitoids from Group 1 show Na2 O ≥ K2 O. The dioritic enclaves hosted by granitoids of Group 1 are clearly less K-rich than the diorites hosted by granitoids of Group 2 (Guimar˜aes et al., 2001). All granitoids studied are metaluminous to slightly peraluminous with A/CNK ≤ 1.1 (Fig. 8). The granitoids from Group 2 are high-K calc-alkaline (Fazenda Nova and Campina Grande) and shoshonitic (Bom Jardim, Toritama and Paje´u – Guimar˜aes and Da Silva Filho, 1998). They show the lowest FeOtot /(FeOtot + MgO) ratios

(Table 2) among the granitoids studied (≤0.60). Group 3 granitoids have high FeOtot /(FeOtot + MgO) ratios (Table 2). They are geochemically similar to the postcollisional ferro-potassic granitoids of Nigeria (Ferr´e et al., 1998). The granitoids of Group 4, the youngest, are SiO2 -rich (>70 wt.%) and show more alkalic character, with higher contents of alkalis, Rb, Zr, Nb, Y, and lower contents of Ba and Sr compared to granitoids of the other groups; they also have high FeOtot /(FeOtot + MgO) ratios, similar to those recorded in granitoids of Group 3. The granitoids of Groups 3 and 4 fall within the ferroan plutons field (Fig. 9) in the FeOtot /(FeOtot + MgO) versus SiO2 diagram of Frost et al. (2001), reflecting a close affinity to relatively anhydrous and reduced magmas, which are common conditions in exten-

36

I.P. Guimar˜aes et al. / Precambrian Research 135 (2004) 23–53

Fig. 6. U–Pb Concordia diagram for the Pereiro, Serra doVelho Zuza plutons.

sional environments. In contrast, granitoids of Groups 1 and 2 plot within the magnesian plutons field (Fig. 9), reflecting hydrous, oxidizing magmas (Frost and Lindsley, 1991). The origin of magnesian granitoids has been interpreted as related to subduction (Frost et

al., 2001). All four groups of the studied granitoids fall within the field of the Caledonian post-collisional granitoids from Ireland and Great Britain (Fig. 9). Chondrite
normalized REE patterns (Fig. 10) show that the REE are comparatively lower in granitoids

Table 2 Representative major and trace element compositions of the studied granitoids Sample

Intrusion Timba´uba TI-33

Itapetim TI-12

IG-01

Tabira IG-36

Paje´u

CT-22

CT-24

PJ-08

PJ-09

Bom Jardim

Toritama

BJ-75

TO-03

BJ-344

Campina Grande TO-04 CG − 23 CG − 21

62.53 0.98 15.50 5.98 2.68 4.56 3.29 3.62 0.33 0.10 0.30

66.10 0.51 15.87 3.79 1.26 2.36 4.76 4.13 0.14 0.05 0.20

66.97 0.65 15.88 3.80 1.16 3.76 4.02 3.09 0.18 0.05 0.74

67.44 0.64 15.98 3.60 1.10 3.32 3.94 3.79 0.17 0.05 0.43

58.96 0.99 17.54 6.96 2.53 2.53 3.17 2.75 0.36 0.11 1.1

58.19 1.01 17.54 7.00 2.62 5.53 3.19 2.80 0.37 0.10 0.90

65.80 0.43 15.41 3.42 1.33 2.56 4.21 6.41 0.33 0.06 0.20

62.12 0.56 14.89 2.03 2.16 3.03 3.91 6.69 0.37 0.07 1.10

55.64 0.91 15.72 5.87 5.06 4.58 3.77 5.71 0.59 0.09 0.56

54.35 1.05 14.96 6.64 6.12 6.13 3.65 4.90 0.74 0.10 0.88

56.28 0.90 16.04 5.92 4.98 5.06 4.02 4.88 0.60 0.09 0.54

56.38 0.89 15.48 6.21 5.16 5.20 3.79 5.17 0.64 0.09 0.62

60.35 0.81 16.32 5.34 2.92 4.21 4.10 4.30 0.52 0.09 0.20

60.40 0.82 16.42 5.29 2.96 4.31 4.24 3.94 0.52 0.09 0.20

Total

100.06

99.70

99.96

99.73

100.15

99.41

100.71

99.95

99.01

99.61

100.11

100.16

99.55

99.90

ppm Ni Cr Zr Hf Y Nb Ta Ba Sr Rb Th La Ce Nd Sm Eu Gd Tb Ho Tm Yb Lu Fe∗

20 130 229 6.5 24 16 0.9 1515 740 130 8.4 56.0 113.0 54.30 10.4 2.37 7.03 0.97 5.49 0.97 2.51 0.36 0.67

15 100 262 7.2 10.1 10.4 1.5 4570 1960 70 8.8 62.0 104.1 41.7 5.9 1.19 3.76 0.47 0.37 0.15 0.80 0.12 0.73

<5 <10 180 5.42 15.6 12 0.94 580 360 158 13.3 38.3 82.03 26.03 4.9 1.26 5.56 0.59

10 20 195 5.30 18.2 14 1.08 650 300 200 16.8 37.7 79.0 27.30 5.1 1.22 5.80 0.63

20 200 260 6.3 23.6 13 <0.5 945 500 110

20 150 275 6.4 25.0 11 <0.5 825 490 90

120 300 270 5.2 16.3 9 0.3 4050 1690 148 4.9 55.97 99.27 46.44 7.0 2.25 4.50 0.56 0.29 0.18 0.87 0.11 0.52

120 310 320 8.0 18.0 15 0.8 3140 1610 140 11.3 89.55 182.4 83.2 13.0 3.65 9.39 1.13

40 70 240 5.2 16.0 20 1.4 3515 900 95 16.0 82.1 122.0 46.0 7.1 1.78

50 90 230 5.8 15.0 19 1.2 3190 940 107 12.9 82.1 137.0 48.0 6.9 1.81

0.50

0.40

0.17 0.83 0.09 0.75

58.7 116.8 38.7 8.6 1.90 6.10 1.10 0.90 0.50 2.20 0.5 0.71

120 225 400 9.9 16.9 16 0.7 3367 1711 150 15.2 100.2 185.9 81.2 12.3 3.31 8.40 1.07 0.49 0.35 1.40 0.20 0.51

160 240 170 12.3 22.8 14 0.6 2475 1810 127 10.4 102.6 206.3 98.2 15.8 4.00 10.9

0.22 1.29 0.13 0.75

72.4 118.5 45.8 7.7 2.00 6.00 0.60 0.80 <0.2 1.90 0.20 0.71

50 190 225 7.5 13.0 11 0.04 2310 820 251 29.7 84.9 155.0 54.0 6.5 1.72

1.16 0.18 0.62

1.11 0.21 0.62

45 165 180 12.0 10 2000 954

0.70

1.46 0.22 0.46

1.70 0.25 0.49

0.45 1.58 0.19 0.52

I.P. Guimar˜aes et al. / Precambrian Research 135 (2004) 23–53

SiO2 TiO2 Al2 O3 Fe2 O3 MgO CaO Na2 O K2 O P2 O5 MnO LOI

37

38

Table 2 (Continued ) Sample

Intrusion Solid˜ao

Serra Branca

Queimadas

Serra do Zuza

Pereiro

Prata

CS-39

SB-05

SB-09

QM-91

QM-107

SZ-222

SZ-231

PE-54

PE-208

SiO2 TiO2 Al2 O3 Fe2 O3 MgO CaO Na2 O K2 O P 2 O5 MnO LOI

69.01 0.19 14.56 1.89 0.34 0.94 4.33 5.53 0.07 0.06 0.70

69.60 0.23 14.72 1.98 0.52 1.13 4.47 5.19 0.16 0.5 0.7

71.45 0.28 14.67 2.42 0.35 1.31 3.46 5.08 0.09 0.04 0.80

72.26 0.49 14.82 3.08 0.74 2.12 3.71 4.56 0.10 0.03 0.60

70.29 0.37 14.33 3.09 0.34 1.42 3.79 5.04 0.16 0.05 0.80

67.18 0.62 14.72 4.86 0.24 2.59 3.62 4.84 0.16 0.06 0.70

72.04 0.24 14.04 2.25 0.26 1.02 3.51 5.52 0.07 0.04 0.30

70.54 0.37 13.70 3.20 0.31 1.30 3.24 5.37 0.12 0.05 1.10

72.47 0.18 13.42 2.33 0.16 0.88 3.37 5.13 0.10 0.04 1.0

69.80 0.35 13.99 3.52 0.33 1.27 3.29 5.72 0.09 0.06 0.70

70.92 0.35 13.90 3.50 0.41 1.00 3.77 5.97 0.06 0.10 0.40

71.92 1.04 13.16 3.37 0.47 1.04 3.50 5.81 0.08 0.09 0.20

Total

99.12

99.20

100.13

99.92

99.95

99.82

99.47

99.55

99.22

99.38

100.38

100.68

<5 80 350 9.5 63 27 <5 1030 140 233 29.5 116.0 237.0 73.0 13.0 1.50

<5 <10 650 17.5 66 36 <5 1933 260 130 16.2 132.0 279.0 110.0 19.5 3.31

20 160 210 5.9 30 24 2.3 590 130 320 32.4 68.9 138.0 52.0 7.7 0.74

ppm Ni Cr Zr Hf Y Nb Ta Ba Sr Rb Th La Ce Nd Sm Eu Gd Tb Ho Tm Yb Lu Fe∗ ∗

20 150 190 5.7 13.7 12 <0.5 7345 1730 110 65.9 118.4 28.8 5.3 1.50 3.00 0.30 0.50 <0.20 1.20 <0.2 0.83

20 30 200 6.1 14.6 16 5210 1475 112 40.3 81.1 23.8 4.4 1.40 2.80 0.50 0.5 0.20 1.10 <0.2 0.77

Fe = Feot/(MgO+FeOt).

15 <10 280 8.2 22.3 16

30 <10 150 5.1 20.8 20

900 205 250 41.7 135.6 209.1 61.0 10.5 0.90 7.40 1.10 1.40 1.60 1.60 0.30 0.86

730 190 320 33.2 65.2 105.4 33.3 6.2 0.60 5.30 0.70 1.20 2.10 1.50 0.3o 0.79

1.5

1.3

1.0

5.70 0.85 0.89

7.00 1.01 0.95

3.62 0.57 0.89

PRT-38

PRT-34

25 180 310 7.8 48 22

42 <10 281 5.8 58 25

<5 200 370 6.2 49 20

5 15 570

<5 <10 460

77 41

902 130 261 28.5 122 233 70.0 12.4 1.14

390 101 343 31.9 68.6 138.0 47 8.9 0.64

915 165 266 38.8 83.6 151.0 64.0 9.9 0.97

430 70 171

1.1

1.5

6.28 0.96 0.93

5.13 0.76 0.91

49 26 0.9 560 110 203 34 222.0 403.1 129.5 17.6 1.3 15.3 1.8 Er = 4.7 0.6 4.0 0.6 0.87

4.19 0.62 0.90

179.22 351.9 120.4 23.4 1.26 14.74 Dy = 12.23 Er = 5.57 4.53 nd 0.88

I.P. Guimar˜aes et al. / Precambrian Research 135 (2004) 23–53

CS-14

I.P. Guimar˜aes et al. / Precambrian Research 135 (2004) 23–53

39

Fig. 7. K2 O vs. SiO2 for the studied granitoids. Fields after Peccerillo and Taylor (1976). (A) Soshonitic series; (B) high-K calc-alkaline series; (C) calc-alkaline series; (D) tholeiitic series.

with SiO2 >69 wt.%. Significant Eu anomalies are not observed in the patterns of Groups 1 and 2, and both groups have fractionated patterns with (Ce/Yb)N > 30. The granitoids of Group 3, except for the Solid˜ao pluton, show significant negative Eu anomalies (Eu* = 0.40–0.67) and (Ce/Yb)N ratios in the 10–16 range. The Solid˜ao granitoids show small positive Eu anomalies and (Ce/Yb)N ratios ranging from 19 to 25 (Fig. 9C), suggesting that they originated from a distinct source. The Group 4 granitoids have higher HREE contents and their patterns are characterized by deep negative Eu anomalies (Eu* = 0.2–0.3), and low (Ce/Yb)N ratios (10–25). Trace element distribution patterns (Fig. 11A–D) show that all granitoids studied have Nb depletion, which decreases slightly from Group 1 to 4. Troughs are also observed at Ti and Sr and tend to increase

in magnitude from Group 1 to 4. The Solid˜ao pluton is an exception, showing small peaks at Sr, which in association with small positive Eu anomalies, suggests an evolution involving feldspar accumulation. All the granitoids studied are enriched in LILE (large ion lithophile elements) compared to HFSE (high field strength elements), which is a general characteristic of calc-alkaline granitoids. Larger negative anomalies in Sr and Ti and distinctively higher contents of Y and Yb are observed in the granitoids from Group 4, similar to those recorded in A-type granitoids (Whalen et al., 1987). The patterns of the granitoids from Groups 1 and 2 display no significant troughs at Sr, less pronounced Ti troughs, and lower Y, Yb, and Nb values, resulting in a trace element distribution pattern characteristic of calc-alkaline arc granitoids.

40

I.P. Guimar˜aes et al. / Precambrian Research 135 (2004) 23–53

Fig. 8. Shand’s Index for the studied granitoids; fields after Maniar and Piccoli (1989).

Discriminant diagrams of Pearce (1996); Pearce et al. (1984) are used here to summarize some of the geochemical trace element features of the granitoids (Fig. 12). Group 1 granitoids have trace element com-

positions which plot within the volcanic arc granitoid field. Granitoids from Group 2 have compositions, which range from the volcanic arc granitoid field to the syn-collision granite field. Granitoids from Group 3

Fig. 9. The compositional range of the studied granitoids in the FeOtot /FeOtot + MgO) vs. weight percent SiO2 diagram. Fields of Ferroan and Magnesian granitoids as well as the field (gray) of Caledonian post-collisional plutons from Ireland and Britain are from Frost et al. (2001).

I.P. Guimar˜aes et al. / Precambrian Research 135 (2004) 23–53

41

Fig. 10. Chondrite- normalized REE patterns (Sun, 1982) of the studied granitoids.

plot within the fields of volcanic arc and within-plate granites. Group 4 granitoids are Nb- and Y-rich, plotting in the within-plate granite field. They are compositionally similar to A-type granites. All the granites studied plot within the post-collision granitoids field of Pearce (1996) diagram. However, as pointed out by Pearce (1996), the post-collision granites are the most difficult to classify, since some have subduction-like mantle sources with many characteristics of volcanic arc granites, and others show within-plate granite character. Interaction between mantle-derived sources and crust tends to move the granite composition towards the volcanic arc field. In the Central Tectonic Domain of the BP, evidence for subduction of oceanic lithosphere are very local (Beurlen et al., 1992), indicating that the Brasiliano Orogeny was mainly ensialic, as proposed by Jardim de S´a (1984), and the arc signature may be inherited from the source.

5.1. Sm–Nd geochemical data Sm–Nd isotopic analyses were made at the Isotope Geochemistry Laboratories, Kansas University, USA. The methodology is described in Appendix A. Results of representative samples are presented in Table 3. The older granitoids (Itapetim, Timba´uba and Tabira) show younger Sm–Nd TDM model ages, similar to those recorded in the metagreywacke and metaigneous country rocks (1.3–1.5 Ga), and higher εNd values (Fig. 13A). These are the only granitoids studied showing Mesoproterozoic TDM model ages. The granitoids of Group 2, including syenites monzogranites, mafic syenite and dioritic enclaves, show Sm–Nd TDM model ages in the 1.8–2.1 Ga range and εNd (at 590 Ma) ranging from −14 to −10. These data suggest that the sources of these granitoids could be a Transamazonian enriched lithospheric

42

I.P. Guimar˜aes et al. / Precambrian Research 135 (2004) 23–53

Fig. 11. Chondrite normalized, trace element abundance diagrams (spidergrams) for representative samples of the studied granitoids. Normalization factors are from Thompson (1982).

mantle (diorites and syenites) and lower crust (granites). Because the syenites, diorites and granites have similar Sm–Nd isotopic signature, the lower crust was probably extracted from a Paleoproterozoic lithospheric mantle, during the initial stage of the Brasiliano Orogeny.

The 570 Ma granitoids show older TDM model ages (2.0–2.4 Ga). The oldest TDM ages were recorded in the Serra Branca pluton, suggesting contribution of an Archean component in its source. The Queimadas magma probably originated by melting of granodiorite from lower crust (Almeida, 1999).

Table 3 Summary of representative Sm/Nd isotopic results ± 2␴

Intrusion

Sample

Nd (ppm)

Sm (ppm)

147 Sm/143 Nd

143 Nd/144 Nd

Itapetim Tabira Timba´uba Paje´u Toritama Bom Jardim Fazenda Nova Campina Grande Solid˜ao Serra Branca Queimadas Prata Pereiro Velho Zuza

ITA-20 CT-24 TI-12 PJ-09 TO-03 BJ-344 FN-02 CG-34 CS-35 SB-03 NA-43 PR-34D PE-208 VZ-236

13.19 45.46 49.90 50.53 61.59 99.68 64.30 116.26 32.81 35.77 58.82 83.50 95.56 37.31

2.46 7.90 8.78 7.83 10.47 16.49 10.17 17.86 5.18 5.79 11.34 13.3 14.93 5.54

0.1129 0.1054 0.1064 0.0937 0.1028 0.1000 0.0956 0.0928 0.0954 0.0979 0.0929 0.09649 0.09448 0.08982

0.512102 ± 26 0.512038 ± 17 0.512053 ± 20 0.511413 ± 14 0.511583 ± 13 0.511599 ± 15 0.511448 ± 26 0.511426 ± 17 0.511440 ± 16 0.511160 ± 20 0.511426 ± 15 0.511319 ± 17 0.511353 ± 13 0.511268 ± 19

εNd(0)

εNd(t)

TDM (Ga)

−10.5 −11.71 −11.42 −23.89 −20.57 −20.27 −23.21 −23.64 −23.37 −28.84 −23.64 −25.74 −25.06 −26.72

−3.60 −4.70 −4.72 −16.02 −13.39 −12.88 −15.48 −13.42 −15.16 −21.29 −15.70 −18.08 −17.25 −18.55

1.42 1.43 1.41 2.07 2.00 1.94 2.06 2.04 2.07 2.49 2.04 2.25 2.16 2.19

Note: 143 Nd/144 Nd normalized to 146 Nd/144 Nd = 0.72190. εNd (0) calculated relative to CHUR(0) = 0.512638. Model ages (TDM ) were calculated according to the single-stage depleted-mantle model of DePaolo (1981). Ages used for εNd (t) are based on U/Pb ages or Rb/Sr age (Prata complex).

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43

Fig. 12. Studied granitoids in the tectonic discriminant diagrams of (A) Pearce et al. (1984) and (B) Pearce (1996).

Granitoids from Group 4 have Sm–Nd geochemistry signature similar to those recorded in granitoids of Group 2.

6. Discussion The granitoids studied record a long period of granitic magmatism (∼100 m.y.) during the Neoproterozoic in the Central Tectonic Domain of the BP. The oldest granitoids are associated with diorites and are low-to-slightly high-K calc-alkaline. However, the contents of K, even in the high-K case, are lower than those recorded in the granitoids of Group 2. To explain the Mesoproterozoic TDM ages recorded in these rocks

it is necessary that isotopic interaction with the host monzogranites occurred. The geochemical and isotopic signature associated with the presence of quartz enclaves in the monzogranites are consistent with an origin by melting of metagreywackes, which originally formed from a mixture of Paleoproterozoic (2.0 Ga) ortho derived crust, Cariris Velhos (1.0 Ga) juvenile material, and small amount of metasediments. The origin of medium to slightly high-K granitoids of Group 1 (Itapetim complex) has been discussed in detail by Guimar˜aes and Da Silva Filho (2000). Calc-alkaline granitoids with ages in the same time span 620–640 Ma have also been recorded in the Pianc´o Alto Br´ıgida belt and in the Sergipano belt of the Southern Domain (Table 4). The granitoids of Group 1 have most of the

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Fig. 13. Nd isotopic composition of the studied granitoids. Isotopic notations, model ages and reference mantle reservoirs are from De Paolo (1988).

features of ACG granitoids of Barbarin (1999), which are the most abundant granitoids in volcanic island arcs and active continental margins associated with K-rich calc-alkaline granitoids. In Cameroon, ages of (630 ± 5) and (620 ± 10) Ma were recorded in recrystallized zircons from syntectonic granitoids and on metamorphic minerals, respectively (Toteu et al., 1990, 1994). In Cameroon this period was interpreted as a stage of convergence, during which a flat-lying foliation was developed under conditions of garnet-kyanite (northern Cameroon) or granulite facies (southern Cameroon) metamorphism and in association with calc-alkaline plutonism (Toteu et al., 2001). Granitoids of Group 1 were deformed

under high-T conditions and were intruded parallel to the country rock foliation, during a flat-lying foliation forming event. This evidence, associated with the crystallization age of these granitoids, a U/Pb sphene age recorded by Leite et al. (2000) in early Neoproterozoic orthogneisses, and a possible correlation with the Cameroon Province, strongly suggests that Group 1 granitoids represent crust reworking during the peak of metamorphism. The stage of convergence ended in Cameroon with general anatexis at 620 Ma (Toteu et al., 2001). In the Central Tectonic Domain of the Borborema Province, rare geochronological data for the 610–595 Ma period (Leite et al., 2000) is an exception. In the Timba´uba area, field relationships suggest that

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45

Table 4 Summary of petrographic, geochemical and isotopic data of the studied granitoids and others from PPB, PABB and SB described in the literature Cristalization age (Ma)

PPB

PABB

SB

Main field and petrographic aspects

Chemistry

640–610

Timba´uba Tabira

Conceic¸a˜ oa Emasb

Coronel Jo˜ao S´ac,d Gl´oriac

Granodiorite epidote-bearing pierced Eo Neoproterozoic low-to-intermediated grade metasediments and orthogneisses. Quartz chunks; mafic enclaves; xenoliths and amphibole-rich clots are common. Mafic enclaves, porphyritic monzo to sienogranites epidote-bearing; intruded in EoNeoproterozoic low-to-intermediated grade orthogneisses

A < CNK; medium-K calc-alkaline; Na2 O/K2 O ≥ 1; Fe# = 0.67–0.75; εNd (t) = −4.0 to −7.1; TDM age = 1.7–1.2 Ga

Sieno to monzogranites and syenites, with enclaves of K-rich diorites and mafic syenites, intruded in the contact zone between low-to-intermediated grade metasediments of Eo Neoproterozoic ages and Paleoproterozoic gneiss-migmatites Ca-pyroxene, bitotite sienogranites, rare mafic enclaves, intruded in orthogneisses of Eo Neoproterozoic (Cariris Velhos) ages (Solid˜ao), Leucocratic biotite granites with rare amphiboles and mafic enclaves/xenoliths (Queimadas, Serra Branca); Amphiboleproxene-syenites (Triunfo). They pierced Paleoproterozoic gneiss-migmatites

A < CNK; high-K calc-alkaline and shoshonitic; K2 O/Na2 O ≥ 1; Fe# 0.49–0.62; εNd (t) = −12.9 to −16.2; TDM age 2.0–1.8 Ga

Itapetim; Conceic¸a˜ o das Criolase Tavaresa

S´ıtios Novosf

610–590

Migmatization

Migmatization Xingo Complexd

590–580

Fazenda Nova Campina Grande Bom Jardim*; Paje´u; Toritama* Caruaru–Arcoverdef

∼570

Solid˜ao Queimadas; Serra Branca Triunfog

Itaporangac

High-K calc-alkaline K2 O/Na2 O ≥ 1; Fe# 0.0.74–0.75; εNd (t) = −4.0 to −7.1; TDM age 1.5–1.2 Ga

A < CNK; alkaline and ultrapotassic (Triunfo); K2 O/Na2 O ≥ 1; Fe# 0.77–0.95; εNd (t) = −21.4 to −15.2 TDM age = 2.6 Ga to −2.0 Ga

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Table 4 (Continued ) Cristalization age (Ma)

PPB

540–512

Prata, Pereiro, Serra do Velho Zuza, Minador, Boqueir˜ao

PABB

Serra Livramentoh ; Ultrapotassic dikesh

SB

Main field and petrographic aspects

Chemistry

Rapakivi granites, associated to subvolcanic rocks, intruded either orthogneisses of EoNeoproterozoic ages and Paleoproterozoic gneiss-migmatites Alkaline amphiboleand pyroxene-bearing syenites to sienogranites

A < CNK; alkaline A-type; K2 O/Na2 O ≥ 1; Fe# 0.87–0.93; εNd (t) = −17.2 to −18.5; TDM age = 2.3–2.0 Ga

Ultrapotassic Fe# 0.74–0.85; εNd (t) = −12.8 to −17.7; TDM age = 2.7–1.8 Ga

Fe = FeOt/(MgO + FeOt). PABB = Pianc´o Alto Br´ıgida Belt; SB = Sergipano Belt; PPB = Paje´u-Para´ıba Belt; (*) = syenites. a Brito Neves et al. (2003). b Sial et al. (1998). c Chaves (1991). d Da Silva Filho et al. (1997). e Brasilino et al. (1999). f Melo et al. (2000). g Ferreira et al. (1997). h Da Silva Filho et al. (1993).

anatexis of the country rocks is contemporaneous with or post-dates intrusions of Group 1 granitoids. In the Sergipano Fold Belt (Southern Domain of the BP) a Rb–Sr age of 600 ± 23 Ma was obtained for the Xing´o Granitic Complex (Santos and Souza, 1988); this age is related to anatexis (Guimar˜aes and Da Silva Filho, 1995). Metamorphic ages of 630 Ma are also recorded in the Tocantins Province, central Brazil. There, they are interpreted as resulting from the collision of the S˜ao Francisco Craton and Amazon Craton and possibly the Paran´a Block (Pimentel et al., 1999). This widespread metamorphic event with age 630–610 Ma, shows that many cratonic blocks were under convergence in the shield of Brazil and Africa, during this period. Within the granitoids of Group 2, the most primitive rocks are mafic syenites and diorites (MgO > 9.0%; Cr ∼ 450 ppm and Ni ∼ 250 ppm), LILE-rich and HFSE-poor, which have fractionated REE patterns and negative εNd values, and Paleoproterozoic (ca. 2.0 Ga) Sm–Nd TDM model ages, similar to the host syenites and monzogranites. The LILE-rich and HFSE-poor signature seems to reflect the source rock signature. Dioritic melts can be generated under temperatures higher than 1050 ◦ C (Rapp and Watson, 1995), which are not expected to be reached in the lower crust dur-

ing orogenic events (Neves and Mariano, 2003). Partial melting of a Paleoproterozoic lithospheric mantle enriched in incompatible elements is a good candidate to explain the source of the mafic syenites and diorites. No direct evidence indicative of an enriched mantle related to the Cariris Velhos event or to Brasiliano/PanAfrican Orogeny in the Central Tectonic Domain of the BP has been found so far. This suggests that the Transamazonian Orogeny was the main, if not the only, event promoting mantle metassomatism under the Central Tectonic Domain of the BP (Guimar˜aes and Da Silva Filho, 1997, 1998; Neves et al., 2000). Monzogranites of Group 2 may have originated by partial melting of Paleoproterozoic lower crust or of Brasiliano—Pan-African new crust derived from a Transamazonian enriched lithospheric mantle. The granitoids of Group 2 have a large amount of mafic enclaves, are coeval with shear zone development, and locally show flat-lying foliation. The lack of evidence for subduction of oceanic lithosphere contemporaneous with the emplacement of the granitoids of Group 2 and the contemporaneity between granitoid intrusions and a period of convergence in the shield of BrazilAfrica suggest that the Central Tectonic Domain of the BP was an intracontinental setting subject to compres-

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sive forces resulting from the convergence of the S˜ao Francisco/Congo, Amazon, and West African Cratons during the Brasiliano/Pan-African orogeny (Neves, 2003). High-K calc-alkaline and shoshonitic granitoids occur in many domains of the Borborema Province (Table 4). Within the Central Tectonic Domain, the high-K calc-alkaline intrusions of Group 2 are associated with shear zones marking the limit of terranes with distinct isotopic signatures (Santos et al., 1997). According to Ferreira et al. (1998), epidote-free, highK granitoids within the Central Tectonic Domain have εNd values ca. −12, while the epidote-bearing ones have εNd values ranging from −1 to −4. It is noteworthy that the Campina Grande Complex is the youngest of the Group 3 plutons, comprises magmatic epidote bearing high-K calc-alkaline granitoids, and has εNd values approximately −12. Other occurrences of epidote bearing high-K calc-alkaline granitoids with ages (580 Ma) and εNd values (−13 to −15) similar to those recorded in the Campina Grande complex, have been described in some of the Esperanc¸a complex intrusions (Sampaio et al., 2003), which is intruded 20 km north of the Campina Grande complex. The isotopic signature of the Campina Grande and Esperanc¸a complexes do not always allow correlation between crystallization of magmatic epidote and εNd values, as proposed by Ferreira et al. (1998). Granitoids of Group 3 were emplaced during an extensional transcurrent, post-collisional event (Ara´ujo, 1997, Guimar˜aes et al., 2000). They may represent the final stage of lateral escape of the Brasiliano compressive event between the S˜ao Francisco—Congo Cratons and initial stage of the extensional collapse of the orogen. The Queimadas, Solid˜ao, and Serra Branca plutons show distinct trace elements. The Queimadas granitoids are within-plate, A-type and low fO2 ilmeniteseries granites (Almeida et al., 1998). The Solid˜ao and Serra Branca granitoids have VAG signature and were crystallized under intermediate-fO2 conditions (Guimar˜aes et al., 2000). The granitoids of Group 3 are associated with a late ductile–brittle stage of deformation, and their geochemical signatures suggest that they represent post collisional alkaline magmatism. They probably originated from a vertically zoned lower crust, with some Archean contribution and during the initial stage of extensional collapse of the Brasiliano Orogen.

47

The Solid˜ao granitoid has a large inherited component and a Paleoproterozoic upper intercept U/Pb zircon age, similar to the Sm–Nd TDM model age. This suggests that these granitoids were generated from a Paleoproterozoic lower crustal source. In the Serra Branca pluton, Sm–Nd TDM model ages (ca. 2.5 Ga: Guimar˜aes and Da Silva Filho, 1997) strongly suggest an Archean component in its source. The migmatitic country rocks of the Serra Branca intrusion have even higher Sm–Nd TDM model ages (2.7 Ga—Guimar˜aes and Da Silva Filho, 1997). The source of the Serra Branca granitoids appears to be a mixture between Paleoproterozoic and Archean crust. All granitoids of Group 3 share some, but not the most, of the petrographic and geochemical characteristics of the Itapetim-type (Almeida et al., 1967) and, except for the rare mafic enclaves observed in the granitoids studied, they can be classified as KCG (K-rich, and K-feldspar porphyritic granitoids), following the classification of Barbarin (1999). The KCG has been described in various geodynamic environments, but it is especially abundant in orogenic belts related to collision at the time when collision is ending. These granites indicate a shift in tectonic regime, rather than a specific geodynamic environment (Barbarin, 1999). KCG can occur either during transition from a compressive regime to a tensional regime or during periods of relaxation that separate periods of culmination within a collision event (Lameyre, 1988). They are distinct from granitoids of Group 2 by having distinct petrographic features, geochemistry signatures, and geochronological data. The granitoids of Group 3 are chronologically correlated with the intrusion of the ultrapotassic Triunfo batholith, in the western part of the Paje´u-Para´ıba belt (Table 4). The granitoids of Group 4 are coeval with a bimodal subvolcanic magmatism and are geochemically similar to A-type granitoids; they share many characteristics of alkaline (but not peralkaline) PAG and KCG granitoids. Peralkaline PAG syenites, with ages within the same time span of granitoids of Group 4, have been described in the western part of the Paje´u-Para´ıba belt, (Sial, 1986) and in the southeastern part of the Pianc´o Alto Br´ıgida belt (514 ± 20 Ma, Rb–Sr, Da Silva Filho et al., 1993). The ages of these granitoids are similar to those recorded in the Mocambo pluton of the Jaibaras Basin, Cear´a State, in the NW part of the Borborema Province. Several pull-apart basins in the Cear´a

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state and in the Central Tectonic Domain document that crustal extension was not locally restricted during early Cambrian times (Fetter, 1999). The basins resulted from widespread continental breakup that took place separating Laurentia and Baltica from Western Gondwana (Bond et al., 1984; Dalziel, 1997; Lieberman, 1997; Brito Neves, 1999a; Brito Neves, 1999b; Oliveira, 2001). A late Neoproterozoic-early Paleozoic rifting episode has also been identified in the central part of Brazil and interpreted as related to the break-up of the Appalachian margin of Laurentia and the protoAndean margin of Gondwana (Pimentel et al., 1996, 1999). In the Fazenda Nova-Toritama area, 40 Ar/39 Ar laser dating of biotite shows cooling ages as young as 545 ± 1.1 Ma (Neves et al., 2000). In the Serid´o Fold Belt, northeastern part of the Borborema Province, the oldest ages were recorded in diorites emplaced at 579 ± 7 Ma (Leterrier et al., 1994). In this domain, Moni´e et al. (1997) and Corsini et al. (1998) reported amphibolebiotite closure ages in the range of 540–500 Ma. In the northwest part of the Borborema Province (Cear´a State) distinct rapid cooling and uplift ages have been reported in distinct areas (Fetter, 1999). The movement of the shear zones in Cear´a state probably ceased before 532 Ma, which is the crystallization age of the postorogenic Mucambo pluton, but the cooling continued until about 522 Ma (Moni´e et al., 1997; Fetter, 1999). The early Cambrian magmatism in the Central Tectonic Domain of the Borborema Province corresponds closely to that recorded by Fetter (1999) for the northwest part of the Borborema Province. It suggests that common tectonic conditions pervaded at least in the central Cear´a Domain and Central Tectonic Domain between 540 and 512 Ma. Post orogenic alkaline granites have been described in Cameroon with ages in the range 550–500 Ma and are interpreted as related to the uplift of the Pan-African Belt (Toteu et al., 2001). Considering that the granitoids of Group 4 represent post orogenic intrusions, their intrusion ages can be considered as the minimum age of the end of the Brasiliano Orogeny within the BP. 6.1. Conclusions and summary More than 100 m.y. of syn- to post-orogenic granitic magmatism has been recorded in the Central Tectonic

Domain of the BP, reflecting the tectonic evolution of the BP during the Brasiliano Orogeny. Geochronological and geochemical data indicate that, within the present study area, four magmatic events occurred between ca. 644 and 512 Ma: (1) Calc-alkaline granitoids chronologically correlated with gabbros, with ages in the 610–644 Ma range. They are related to the peak of metamorphism of a flat-lying event associated with the convergence between the S˜ao Francisco/Congo Craton and Western African Craton. They have higher εNd values compared to the other granitoids studied, and based on geochemical data they are low to medium-K and slightly high-K calc-alkaline. We suggest that these granitoids originated by partial melting of early Neoproterozoic (Cariris Velhos) crustal rocks. Field relationships show that migmatization took place during or after intrusion of the Timba´uba granitoids, and finished before the intrusion of the 590 Ma granitoids. The period between 620 and 590 Ma may be, therefore, associated with extensive migmatization in the region. In the South Domain of the Borborema Province, intense migmatization occurred in this same time span. (2) Shoshonitic and high-K calc-alkaline granitoids, with U/Pb zircon ages of 590–581 Ma, mark the transition between the flat-lying event and the transcurrent event. They resulted from partial melting of an enriched Paleoproterozoic mantle and are associated with large volumes of K-rich diorite. High-K calc-alkaline granitoids constitute large intrusions in the BP, suggesting that 580–590 Ma was a period of significant magma accretion to the crust in the Borborema province. (3) Alkaline post-collision granites, with U–Pb ages of ca. 570 Ma, were generated by partial melting of a granodioritic lower crust, marking the final stage (lateral escape?) of the Brasiliano—Pan-African orogen and beginning of the uplift. Synchronously, intrusions of ultrapotassic rocks occured in the Pianc´o Alto Br´ıgida belt and the Texeira high. (4) A-type post-orogenic extension-related granites associated with subvolcanic bimodal magmatism formed between ca. 540 and 512 Ma. They are contemporaneous with the deposition of small sedimentary basin from North and Central Tec-

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tonic Domain of the Borborema Province, probably resulting from an event of rifting related to a widespread continental break up that took place during separation of Laurentia and Baltica from Gondwana.

Acknowledgements We are thankful to Sergio Pacheco Neves and Roberto Weinberg for their helpful in improving an early version. Constructive journal reviews by Jaziel S´a and Marcio Pimentel are appreciated. To Allen Fetter and Marianne Kozuch for their help with isotope analyses at the Isotope Geochemistry Laboratories, Kansas University, USA. This study was financially supported by CNPq (Brazilian National Research Council) Grant no. 475693/2001–9. I.P.G. is grateful for financial support given by CAPES (Coordenac¸a˜ o de Aperfeic¸oamanto de Pessoal de Ensino Superior – Gov. do Brasil), through the Grant BEX0742/96, for a post-doctoral program at Kansas University – USA. WRVS acknowledges support from NSF Grants EAR9117594, -9316047, and -9614473, which have supported his work in Brazil and partially supported operation of the Isotope Geochemistry Laboratory at the University of Kansas.

Appendix A A.1. Sm–Nd isotope determinations Rock powders for Sm/Nd analyses were dissolved and the REE were extracted using the general methods of Patchet and Ruiz (1987). Isotopic compositions were measured with a VG Sector 5-collector mass spectrometer. Sm was loaded with H3 PO4 on a single Ta filament and typically analyzed as Sm+ in the static multicollector mode. Nd was also loaded with H3 PO4 on a single Re filament having a thin layer of AGW-50 resin beads and analyzed as Nd+ using the dynamic mode. The 100 ratios were collected with a 1 V 144 Nd beam; this typically yields internal precision of 10–20 ppm. External precision based on repeated analyses of an internal standard is comparable at ±30 ppm (1δ); all analyses are adjusted for instrumental bias determined by measurements of a internal standard for periodic adjustment

49

of collector positions; on this basis our analyses of La Jolla Nd average was 0.511870 ± 0.000009. A.2. U/Pb determinations Samples were crushed and heavy minerals were concentrated using a Pyramid table, Franz magnetic separator, and heavy liquid separations. Zircon fractions were hand picked and air abraded (pyrite) for 2 h. After that, they were washed with HNO3 . Mineral grains from different fractions were selected using a binocular microscope and loaded into savillex PFA teflon micro-capsules. They were washed three times in ultraclean 7N HNO3 , followed by small volume of mixed 205 Pb/235 U spike. Zircons were dissolved using procedures modified after Krogh (1973, 1982) and Parrish (1987). Isotopic ratios were measured in static mode using a VG Sector single-collector mass spectometer using the Daly detector. Pb isotopic compositions were analysed on single Re filaments using silica gel and phosphoric acid. U was loaded in single Re filament with phosporic acid and a layer of colloidal carbon. Pb isotopic ratios were corrected for a mass fractionation of 0.18 ± 0.15% per amu and U isotopic ratios were corrected for a mass fractionation of 0.30 ± 0.15% per amu. Those measurements for Pb and U taken in Faraday mode were corrected for 0.12 ± 0.05% and 0.0 ± 0.15%, respectively. Uncertainties in U/Pb ratios due to uncertainties in fractionation and mass spectrometry for typical analyses are ±0.5%. Zircon data were regressed using the ISOPLOT program of Ludwig (1993). Uncertainties in concordia intercept ages are given at the 2-sigma level.

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